It is indispensable for quality control to measure the screw thread elements, such as pitch, lead and height, and inspect the surface flaw of screw thread. The measurement has hitheto been performed by use of contact instruments, such as various gauges, and various attempts have been made for automatization of measurement, but the inspection of surface flaw depends almost on visual inspection not to have till now realized a practical automatic inspection method and apparatus. Accordingly, the present invention is intended to propose a method and an apparatus for automatically inspecting the surface flaw at the screw.
The prior art of such inspection apparatus has been disclosed in the Japanese Patent Kokai No. 54-150163. Next, explanation will be given on the above in accordance with FIG. 1. A screw 10, as shown in FIG. 1, is irradiated with the light in the oblique direction with respect to the screw 10 and not-perpendicular with respect to the axis 11 thereof or the enveloping surface 12, so that one flank 13 is overall shaded and the other flank 14 is in part shaded, whereby when an image of screw 10 is picked up perpendicularly to the axis 11 or enveloping surface 12, optical information is obtainable which shows the distribution of the reflected light whose brightness at the shaded portion has a low level. Also, in a case where no flaw exists on the surface of screw thread, the optical information shows a regular bright-dark alternating pattern as shown in FIG. 2. On the contrary, for example, in a case where the flaw at the not-machined root exists as shown in FIG. 3, the light receiving flank 14 is irradiated overall to the root side, but the reflection brightness toward an image pickup apparatus is low at the corner between the flank 14 and the flat root 16, so that the brightness of the reflected light distribution is as shown in FIG. 4, in which the regular bright and dark pattern as in FIG. 2 is not obtained, but an irregular bright and dark pattern is obtained as shown in FIG. 4. Although such pattern makes it possible to detect various surface flaw, the inspection apparatus of the prior art, as understood from the aforesaid measurement principle, can detect large flaw in the thread elements, such as improper height and pitch. Hence, it has been absolutely impossible to inspect a minute surface flaw at the screw thread, such inspection having conventionally inevitably relied on the visual inspection and being the object for inspection by the present invention.